A multiple birth occurs when more than one fetus is carried to term in a single pregnancy. Different names for multiple birth are utilized, depending on the number of offspring. Common multiples are two and three, known as twins and triplets respectively. Twins, triplets and other multiple births occur to varying degrees in most animal species, although the term is most applicable to placental species.

There are two common types of multiple births, fraternal (dizygotic) and identical (monozygotic). Identical siblings arise where one egg is fertilized and the resulting zygote splits into more than one embryo. Identical siblings therefore have the same genetic material. Fraternal siblings result from the fertilization and implantation of more than one egg, so fraternal siblings are not genetically identical, and instead have the coequal genetic similarity any other full siblings do. In some multiple births, it is possible for a combination of these (for example, a set of triplets may have one fraternal baby from one egg, plus two identical twins from a second egg). This is called a polyzygotic birth.

The most common form of human multiple birth is twins (two babies), but the typical order of multiple births in other placental species differs dramatically. Some species give birth to multiple offspring as a matter of course and the resulting group of offspring is called a litter.

Terms used for the order of multiple births are largely derived from the Latin names for numbers. Two offspring (twins) is the most common form of multiple births to nine offspring (nonuplets) being the rarest form of multiple births.